1. Field of the Invention
The present invention relates generally to a compressor and, more particularly, to a variable displacement compressor which can optimize a compression ratio of a hydraulic fluid and adjust a compressor displacement.
2. Description of the Related Art
A variable displacement compressor which can optimize a compression ratio of a hydraulic fluid and adjust a compressor displacement by varying a stroke of a piston in the compressor has been well known in the art, and is commonly used as a refrigerant compressor in an air conditioning system of, e.g., a vehicle. One example of conventional variable displacement compressors is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-91366 (JP-A-7-91366), and FIG. 5 shows the main parts of this example.
As shown in FIG. 5, the variable displacement compressor disclosed in JP-A-7-91366 includes a housing 101, a drive shaft 102 rotatably supported in the housing 101, a rotor element 103 fixed on the drive shaft 102 for rotation together with the drive shaft 102, a swash plate 104 as a cam plate carried on the drive shaft 102, and plural single headed pistons 106 (only one piston 106 is shown) housed inside respective cylinder bores 101a (only one cylinder bore 101a is shown). The swash plate 104 is provided with a center through hole 105 through which the drive shaft 102 extends, and is operatively connected with the rotor element 103 through a joint mechanism 109. The swash plate 104 is also operatively engaged with the pistons 106 through respective pairs of hemispherical shoes 108 to reciprocate the pistons 106 in the cylinder bores 101a due to the rotation of the swash plate 104.
The joint mechanism 109 includes a guide pin 110 formed on the swash plate 104 to project obliquely from a front surface 104a (or a left surface in the figure) of the swash plate 104. The guide pin 110 is substantially located along a certain radius line "D" of the swash plate 104, the radius line "D" being specified so that one piston 106 is positioned at a top dead center thereof when the swash plate 104 is engaged with this piston 106 along the radius line "D". The guide pin 110 is provided at the distal end thereof with a generally spherical head 110a.
The joint mechanism 109 also includes a support arm 111 formed on the rotor element 103 to project from the upper end portion of the rotor element 103 toward the swash plate 104. The support arm 110 is provided with a guide hole 111a, the axis of which extends obliquely in relation to a rotation axis "L" of the drive shaft 102. The guide pin 110 is inserted into the guide hole 111a, and the generally spherical head 110a of the guide pin 110 is slidably engaged with the inner wall surface of the guide hole 111a.
The center through hole 105 of the swash plate 104 is provided with a support portion 105a on the inner wall surface of the center hole 105 to provide a slidable support for the swash plate 104 on the drive shaft 102. The support portion 105a is defined at a certain position opposite to the joint mechanism 109 in relation to the rotation axis "L" of the drive shaft 102. The support portion 105a is also formed at an apex of the inner wall surface of the center hole 105, and includes a generally arcuate surface extending about a lateral axis "S" in a vertical section as shown in FIG. 5.
The swash plate 104 is operatively connected through the joint mechanism 109 and the rotor element 103 to the drive shaft 102 and thus can rotate together with the drive shaft 102 in the housing 101. The rotating motion of the swash plate 104 due to the rotation of the drive shaft 102 is converted to the reciprocating motion of the pistons 106 in the cylinder bores 101a in a direction parallel to the axis "L" of the shaft 102 through the sliding engagement in the shoes 108, so that a hydraulic fluid, such as a refrigerant, is intermittently compressed in the cylinder bores lOla.
The swash plate 104 can also be shifted on the drive shaft 102 to change the inclination of the swash plate 104 relative to the rotation axis "L" of the shaft 102, under the slidable guide for the spherical head 110a of the guide pin 110 in the guide hole 111a of the support arm 111 in the joint mechanism 109, and also under the slidable support for the swash plate 104 on the drive shaft 102 at the support portion 105a in the swash plate center hole 105. When the inclination of the swash plate 104 is changed, the stroke of each piston 106 is varied, and thereby the compression ratio of the hydraulic fluid is optimized or the displacement of the compressor is adjusted.
During the operation of the compressor, the piston 106 is subjected to a compressive load "K" due to the compression of the hydraulic fluid or refrigerant gas, and the compressive load "K" is applied to the inner wall surface of the guide hole 111a, on a front side (or a left side in the figure) away from the swash plate 104, through the shoes 108, the swash plate 104 and the head 110a of the guide pin 110. Then, the reaction force "F" is applied to the head 110a of the guide pin 110 from the inner wall surface of the guide hole 111a. In this respect, the guide hole 111a extends obliquely relative to the axis "L" of the drive shaft 102, that is, one open end of the guide hole 111a opposed to the shaft 102 is located nearer the front surface 104a of the swash plate 104 than another open end of the guide hole 111a. Therefore, the reaction force "F" applied to the guide pin 110 acts to bias the swash plate 104 in a direction "f" toward the joint mechanism 109 in relation to the drive shaft 102. In this manner, during the operation of the compressor, the support portion 105a in the center hole 105 of the swash plate 104 is continuously urged onto the outer circumferential surface of the drive shaft 102.
When the displacement of the compressor is adjusted, the swash plate 104 is shifted in a sliding manner on the drive shaft 102 to change the inclination of the swash plate 104 relative to the shaft 102, while the support portion 105a of the swash plate 104 is urged onto and slidably engaged with the outer circumferential surface of the shaft 102. In this shifting motion, the swash plate 104 is also tilted or rotated about the lateral axis "S" of the arcuate surface of the support portion 105a.
As described above, in the compressor disclosed in JP-A-7-91366, the swash plate 104 is supported on and guided along the drive shaft 102 through the slidable support due to the support portion 105a in the center hole 105. This structure serves to reduce the number of parts, such as a sleeve slidably provided on the shaft and pivot pins formed on the sleeve to pivotably support the swash plate, which has been used for the slidable support between the drive shaft and the swash plate in the other conventional compressor (e.g., a compressor disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-159464), and thereby serves to decrease the production cost and to ease stock management.
On the other hand, in the compressor disclosed in JP-A-7-91366, the support portion 105a of the swash plate 104 is firmly urged and contacted under pressure onto the outer circumferential surface of the drive shaft 102 during the operation of the compressor. In this respect, the arcuate surface of the support portion 105a is formed by machining or milling the center hole 105 of the swash plate 104, and thereby a machined metallic surface is exposed on the support portion 105a. The machined surface of the support portion 105a has a relatively large coefficient of friction, and thus tends to increase a friction force caused between the drive shaft 102 and the support portion 105a during the shifting motion of the swash plate 104 to adjust the displacement. Consequently, it is difficult to smoothly shift or tilt the swash plate 104 on the drive shaft 102, which may deteriorate the responsibility of the adjustment of the compressor displacement.